A popular Medium Access Control (MAC) method for wireless local area networks is a Carrier Sense Multiple Access Collision Avoidance (CSMA/CA). This method works by measuring the time since the last activity on the shared communication channel in equal length contention slots. Each transmitting unit in the network generates a random number, then counts the number of contention slots until the number is reached. At that point the transmitting node can grab the channel and other nodes must suspend their count until the channel is free again.
If, by chance, two nodes generate the same or random number and, thus, collide, they determine that the packet transfer was unsuccessful through the acknowledgement process. In this case the nodes generate a new random number to start counting slots from zero to the next time the channel is ready to transmit.
In several wireless networking protocols, such as the 802.11 wireless protocol, a transmitting node can reserve a particular slot through transmitting at that time. In this case, a node can reserve a set amount of time for the transmission cycle of the particular data.
For example, in the 802.11 protocol, the transmitting nodes reserve a particular transmission length of time by initiating a Request To Send (RTS) packet. Inside the packet, the RTS contains a field that reserves a particular amount of time for the transmission of the data. The receiving node receives the RTS and the associated reservation of time.
The receiving node then sends a Clear To Send (CTS) response. In the 802.11 protocol, the CTS also contains the reservation parameter that was sent in the RTS. In this manner, all nodes in range of the transmitting node receive the RTS and the associated reservation. Additionally, all the nodes within the transmitting range of the receiving node receive the CTS with the corresponding reservation. As such, all the nodes within the transmission range of both the transmitting node and receiving node will know the appropriate reservation for the channel. In this case all the nodes within range of both the transmitting node and receiving node will know the appropriate reservation. From this manner all the nodes within receiving range can determine not to transmit and interrupt the data link between the transmitting node and the receiving node. In this manner, the reservation effectively quiets communication around the transmitter and the receiver for the reserved period. However, a proper prioritization may not be accomplished by such a system. The backoff times may tend to overlap, and transmissions from a more important wireless network device may get put back when the random backoff from a not as important is less than the backoff in wireless network device having a more important transmissions.